Laser distance measuring devices have been widely used in construction, interior decoration and other fields due to their high accuracy of measurement. They have a measuring range up to tens of meters and are usually designed to be handheld devices. The general principles of measurement are as follows: an emitter emits an intensity modulated measuring beam to an object to be measured; the measuring beam is reflected or scattered by the object and is picked up by a photoreceiver and the distance from the object to be measured is determined based on the phase position of the modulated measuring beam relative to the emitter.
In the prior art, a filter device with a predetermined bandwidth is usually arranged in front of the photoreceiver. The modulated measuring beam emitted by the emitter has a bandwidth less than the bandwidth of the filter device so that both the measuring beam reflected by the object and some disturbing light, or noise, within the bandwidth of the filter is received by the photoreceiver, while most of the disturbing light is filtered out by the filter device. However, when used in an outdoor strong light environment, the intensity of the disturbing light in the background increases. As a result, the disturbing light received by the photoreceiver increases and the signal to noise ratio (the ratio of the reflected measuring beam signals received by the photoreceiver to the disturbing light signals received by the photoreceiver) decreases, which results in both the ranging capability and effective measurement distance of the laser distance measuring device decreasing when compared to the indoor measurement capabilities. To improve the ranging capability of the laser distance measuring device under a bright or strong light environment, the bandwidth of the filter device can be narrowed to filter out more of the noise signals (i.e., the disturbing light signals), however, additional useful portions of the measuring beam signal will also be filtered out if the bandwidth of the filter device is narrowed. As a result, when the useful portion of the measuring beam signal received by the photoreceiver is reduced along with the noise, the signal to noise ratio is not effectively improved. Further, due to security concerns, there is a limit to the emitting power of the laser distance measuring device. Currently, the upper limit of the maximum power is 1 mW. Accordingly, the ranging capability of the laser distance measuring device cannot be improved by arbitrarily increasing the emitting power of the emitter.